1. Field of the Invention
The present invention relates generally to a semiconductor structure and a process thereof, and more specifically to a semiconductor structure and a process thereof that forms a metal oxide layer on a work function metal layer.
2. Description of the Prior Art
Poly-silicon is conventionally used as a gate electrode in semiconductor devices such as metal-oxide-semiconductors (MOS). But with the trend towards scaling down the size of semiconductor devices, conventional poly-silicon gates face problems such as lower performances due to boron penetration and unavoidable depletion effect. This increases the equivalent thickness of the gate dielectric layer, reduces the gate capacitance, and worsens the driving force of the devices. Therefore, work function metals that are suitable to be used as high-K gate dielectric layers are employed to replace the conventional poly-silicon gate to be the control electrode.
Generally, methods of forming a metal gate by replacing a conventional polysilicon gate with a work function metal include: a sacrificial gate is formed on a substrate; a spacer is formed on the substrate beside the sacrificial gate; a source/drain region is formed and automatically aligned by using the spacer; an interdielectric layer is disposed and planarized on the substrate; the sacrificial gate is removed to form a recess, and then a work function metal layer, a barrier layer and aluminum are sequentially filled into the recess to form a metal gate.
As sizes of semiconductor components are reduced, material layers such as a barrier layer having large enough thicknesses to prevent the aluminum from diffusing downwards would be filled into the recess after the work function metal layer is filled. Thus, part of the volume of the recess and the opening size of the recess are occupied, so that difficulties for filling the recess with aluminum occur. Furthermore, as the sizes of the semiconductor components are reduced, the volume and the exposing surface area of aluminum shrink, the contact resistance between the aluminum and a contact plug formed above increases. As the semiconductor components are formed precisely, the electrical demand is more critical. How to improve the work function values of the semiconductor components becomes an important issue.